Optical articles, such as ophthalmic lenses, are typically manufactured and sold with various coatings that impart or improve desired optical and aesthetic characteristics of the optical article. Hard-coatings and anti-reflective coatings are two coatings that are often employed in conjunction with one another on a single surface of an optical article. Hard-coatings may be formed by dip or spin coating and can be based upon organic siloxane chemistry. The hard-coating can provide improved abrasion resistance, improved mechanical durability and compatibility with anti-reflective coatings.
Anti-reflective coatings reduce reflection off the front and/or back surface of ophthalmic lenses and therefore are desirable for creating eyeglasses with improved light transmission, visibility, and aesthetics. Typically, such anti-reflective coatings are applied as a series of layers of different materials having different refractive indices. This system of layers is often referred to as a “stack.” Anti-reflective coatings are typically applied to optical surfaces through physical vapor deposition techniques and machines, such as sputter coating systems and machines.
FIG. 1 shows a cross-sectional view of an ophthalmic lens 10 having a bulk lens material 8 with a front convex side 12 and a back concave side 14. The lens 10 may have a hard-coating 16 formed on a convex side 12 and on a concave side 14. The hard-coating 16 is typically in the order of several microns thick. A lens manufacturer may provide lenses with thermally cured hard-coatings on both the convex side 12 and the concave side 14 of the lens 10.
An anti-reflective coating 18 can be applied over the hard-coating 16 on both the convex side 14 and the concave side 14 of the lens 10 either by the manufacturer or by a prescription lab. The convex side 12 and the concave side 14 anti-reflective coating or stacks 18 are formed in an identical or substantially identical manner. In other words, the convex side 12 and the concave side 14 anti-reflective coatings 18 are formed of the same materials with the same number of layers having the same nominal thicknesses.
In the prescription lab, the concave side 14 of the lens 10 is often ground or surfaced to generate the desired curvature and prescription from a semi-finished ophthalmic lens. This process removes the factory applied back side thermally cured hard-coating. In order to restore some of the abrasion resistance and mechanical properties to the ground concave side of the lens lost by removal of the thermally cured hard-coating applied by the lens manufacturer, the prescription lab will most often apply an ultraviolet, UV, cured hard-coating. The above-described concave side anti-reflective coating 18 is then applied over the concave side UV cured hard-coating 16.
One problem often observed with conventional anti-reflective coated ophthalmic lenses formed as described above, is that the concave side anti-reflective coating is susceptible to crazing or cracking. The forces and pressures exerted on an ophthalmic lens during the lens edging process and during insertion of the edged lens into a lens frame are thought to lead to crazing of the concave side anti-reflective coating.
What is needed in the art is an anti-reflective coating system and method for applying the same that effectively eliminates or has increased resistance to crazing and other forms of stress induced damage.